The search for new transparent electrode (TE) materials that would replace the transparent conducting oxides (TCOs), e.g., indium tin oxide (ITO), zinc oxide (ZnO) and fluorine-doped tin oxide (SnO2:F), commonly employed for various opto-electronic applications, has been the focus of a series of extensive studies [1].
This quest has been motivated by two main objectives. First, the ever-increasing price of indium and the deposition process drove the continuous effort to find inexpensive substitutes with similar unique physical properties of high transparency along with low sheet resistivity, and second, the inability of the TCOs family in meeting the special requirements of the new generation devices. For instance, TCOs are not fully compatible with novel plastic based flexible opto-electronic devices. In addition to inexpensive production process, new TE materials have, therefore, to possess a variety of properties such as flexibility and low temperature implementation, while maintaining their physical properties comparable to those of traditional TCOs.
Recently, a few new promising TE candidates were suggested. The majority of those are based on conducting polymers, in addition to nanoscale materials such as carbon nanotubes, graphene flakes, hybrid metal-polymers films [2], nanoscale metallic gratings [3-6], and random networks of metal nanowires (NWs) made of copper [7, 8], silver [9-11] and gold-silver [12,13]. These TEs can potentially meet the new generation device requirements due to their inexpensive deposition processes as well as their flexibility and large-scale implementation. Moreover, their production does not include a high vacuum deposition step, as do traditional TCOs. In addition, the traditional deposition methods are often incompatible with the emerging organic semiconductor based devices such as light emitting diodes or transistors due to sensitivity of the organic material to the sputter-coating process. This is basically true when the TC is required as a top electrode in the final phase of the device production. Some of the new TE materials, such as metallic NWs, have an additional key advantage—their solution-based deposition implementation.